The objectives of this project are to determine in the renal proximal tubule the extent to which metabolic conversion contributes to lactate reabsorption, and to characterize lactate reabsorptive transport. Lactate reabsorption in mammalian proximal tubule occurs against an electrochemical gradient, but could be due to conversion of lactate to CO2, glucose, etc. Unidirectional 14C-lactate fluxes will be measured in isolated perfused Thamnophis (Garter snake) proximal tubule. Thanophis was chosen since lactate oxidation rate should be lower in tissue from a snake, an animal which relies heavily on glycolysis for energy production. With a lower oxidation rate, lactate transport and metabolism may be more easily separated. Preliminary experiments revealed a greater lactate reabsorptive flux than backflux at 25 degrees, not explicable by the electrochemical gradient; both fluxes are inhibited at 5 degrees. During lactate reabsorption, 98% of the 14C-lactate reabsorbed was recovered in the bath as nonvolatile 14C, as chromatography showed that 95% of the 14C in the bath was lactate. These results suggest that lactate reabsorption occurs via active transport with very little metabolic conversion. Future experiments will aim at confirming these preliminary results and then characterizing lactate reabsorption with respect to kinetics (Vmax and Km), specificity, and possible coupling to Na+ reabsorption or H+ secretion. This project will clarify the mechanism of renal lactate reabsorption, and is valuable especially since reabsorption of this important substrate of oxidation in mammalian kidney has not been thoroughly studied previously because of the difficulty of seperating the roles of metabolism and transport.